Method of Producing a MEMS Device

ABSTRACT

A method of producing a MEMS device removes the bottom side of a device wafer after its movable structure is formed. To that end, the method provides the device wafer, which has an initial bottom side. Next, the method forms the movable structure on the device wafer, and then removes substantially the entire initial bottom side of the device wafer. Removal of the entire initial bottom side effectively forms a final bottom side.

The present application is a continuation application of U.S.application Ser. No. 12/129,283 filed on May 29, 2008 which is acontinuation of Ser. No. 10/914,576 filed on Aug. 9, 2004, all of whosedisclosures are incorporated herein, in their entirety, by reference.

RELATED PATENT APPLICATIONS

This patent application is related to co-pending U.S. patent applicationSer. No. 10/914,575, entitled, “MEMS DEVICE WITH NON-STANDARD PROFILE”,filed on Aug. 9, 2004, published as U.S. Patent Application PublicationNo. 2006-0027885 and owned by Analog Devices, Inc., the disclosure ofwhich is incorporated herein, in its entirety, by reference.

FIELD OF THE INVENTION

The invention generally relates to microelectromechanical systems (MEMS)and, more particularly, the invention relates to methods of producingMEMS devices.

BACKGROUND OF THE INVENTION

Microelectromechanical systems (“MEMS,” also referred to as “MEMSdevices”) are a specific type of integrated circuit used in a growingnumber of applications. For example, MEMS currently are implemented asgyroscopes to detect pitch angles of airplanes, and as accelerometers toselectively deploy air bags in automobiles. In simplified terms, suchMEMS devices typically have a very fragile movable structure suspendedabove a substrate, and associated circuitry (on chip or off chip) thatboth senses movement of the suspended structure and delivers the sensedmovement data to one or more external devices (e.g., an externalcomputer). The external device processes the sensed data to calculatethe property being measured (e.g., pitch angle or acceleration).

Although they are relatively small, there still is a continuing need toreduce the size of MEMS devices and other types of integrated circuits.For example, in the cell phone industry, engineers often attempt toreduce the profile of internal application specific integrated circuits(ASICs) that have circuitry only. A reduction in the ASIC profiledesirably can lead to a corresponding reduction in the overall size ofthe cell phone. To those ends, many in that field use conventionalsubstrate thinning processes (e.g., backgrinding processes) to thin thesubstrates of many types of ASICs.

Undesirably, the prior art does not appear to have a similar solutionfor MEMS devices. Specifically, prior art substrate thinning techniquesmay damage the fragile MEMS movable structure. For example, prior artbackgrinding processes (commonly used for integrated circuits withoutstructure) require that the top side of the ASIC be secured to asupporting surface so that the bottom side may be exposed forbackgrinding processes. The top side of a MEMS device, however, has thefragile MEMS structure. Accordingly, the MEMS structure would be crushedif backgrinding were used to thin the substrate of a MEMS device.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a method of producing aMEMS device removes the bottom side of a device wafer after its movablestructure is formed. To that end, the method provides the device wafer,which has an initial bottom side. Next, the method forms the movablestructure on the device wafer, and then removes substantially the entireinitial bottom side of the device wafer. Removal of the entire initialbottom side effectively forms a final bottom side.

The initial bottom side may be formed by a number of methods, such as byusing backgrinding processes. Alternatively, a chemical may be used tochemically etch the initial bottom side. In that case, the structure issealed from the chemical. Moreover, the movable structure may be formedon or by the top surface of the device wafer. In some embodiments, thedevice wafer is fixed to a protective film having a clearance hole thatseals the structure. Consequently, the structure may remain protectedfrom debris or other external objects that could adversely impact itsoperation.

The method also may couple a cap wafer to the device wafer, thusencapsulating the structure. In some embodiments, the method singulatesthe cap wafer before removing the initial bottom side. In alternativeembodiments, the method singulates the cap wafer after removing theinitial bottom side.

In accordance with another aspect of the invention, a method ofproducing a MEMS device provides a device wafer having an initial bottomside and movable structure. Next, the method removes substantially theentire initial bottom side of the device wafer to form a final bottomside. The operation of the movable structure is substantially unaffectedby removal of substantially the entire initial bottom side of the devicewafer.

In accordance with another aspect of the invention, a method ofproducing a MEMS device provides a device wafer having an initial bottomside and movable structure, and fixes the device wafer to a protectivefilm having a clearance hole. The device wafer is fixed to theprotective film in a manner that seals the movable structure within theclearance hole. After the device wafer is fixed to the protective film,the method removes substantially the entire initial bottom side of thedevice wafer to form a final bottom side. Among other ways,substantially the entire initial bottom side of the device may beremoved by backgrinding or chemically etching processes.

After the final bottom side is formed, the film may be removed. It thenmay be coupled with a cap wafer to form a substantially completed MEMSdevice.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and advantages of the invention will be appreciated morefully from the following further description thereof with reference tothe accompanying drawings wherein:

FIG. 1 schematically shows a MEMS device that may be produced inaccordance with illustrative embodiments of the invention.

FIG. 2 shows a generic process for thinning a MEMS wafer in accordancewith illustrative embodiments of the invention.

FIG. 3 schematically shows a plan view of a working surface that maysecure a wafer having MEMS devices during thinning processes.

FIG. 4 schematically shows a plan view of a film frame that may be usedby illustrative embodiments of the invention to in part form the workingsurface shown in FIG. 3.

FIG. 5 schematically shows a plan view of the film frame of FIG. 4having a layer of film for securing a MEMS device.

FIG. 6 shows a more specific process for thinning a MEMS wafer having adiced cap wafer.

FIG. 7 schematically shows a cross-sectional view of a MEMS wafer with adiced cap wafer on a frame with pierced film as shown in FIG. 5.

FIG. 8 shows a more specific process for thinning a MEMS wafer having anundiced cap wafer.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In illustrative embodiments, a method of producing a MEMS device thinsthe MEMS substrate after MEMS structure is formed. In fact, the methodcan be used with either capped or uncapped MEMS devices. Among otherways, the method may use a chemical etch or backgrinding processes tothin the substrate. Details of various embodiments are discussed below.

FIG. 1 schematically shows a generic MEMS device 10 that may be producedin accordance with illustrative embodiments of the invention.Specifically, the MEMS device 10 shown includes a device wafer 11 havingmovable structure (not shown herein but shown in incorporated patents,noted below) suspended over a substrate. Conventional processes may beemployed to form the movable structure. For example, the structure maybe formed by conventional surface micromachining (“SMM”) techniques. Asknown by those skilled in the art, surface micromachining techniquesbuild material layers on top of a substrate using additive andsubtractive processes. In such cases, the MEMS structure may beconsidered to be formed slightly above or on the top surface 12 of asilicon wafer.

As a further example, the structure may be formed by etching materialfrom the top wafer of a silicon-on-insulator wafer (“SOI wafer,” notshown). In such case, the MEMS structure may be considered to be formedsubstantially flush with or below the top surface 12 of a silicon wafer.Of course, other types of processes may be employed to form the MEMSstructure.

In illustrative embodiments, the device wafer 11 is considered to have atop surface 12 and a bottom surface 14. The top surface 12 may beconsidered to include a flat surface and the MEMS structure (e.g.,movable mass and supporting structure), while the bottom surface 14 maybe the bottom surface of the substrate. As noted above, during the MEMSproduction process, the bottom surface 14 (also referred to as “finalbottom surface 14”) is formed by thinning the original bottom surface ofthe device wafer 11. In other words, the bottom surface 14 of thefinished MEMS device 10 schematically shown in FIG. 1 is formed byremoving substantially the entire bottom surface 14 of the substrateabove which the structure originally was formed. Details of the thinningprocess are discussed below with reference to FIGS. 2-8.

To protect the fragile MEMS structure, the MEMS device 10 also has a cap16 secured to the device wafer 11 via a glass bonding layer. Inillustrative embodiments, the cap 16 hermetically seals the structure(i.e., the cap 16 encapsulates the structure) from the environment. Asfurther environmental protection, conventional processes also may mountthe entire MEMS device 10 within a package. As noted above, however, theMEMS device 10 may omit the cap 16. In that case, the MEMS device 10preferably is within a package that can sufficiently protect the MEMSstructure from environmental contaminants, such as dust and moisture.Accordingly, discussion of a MEMS device 10 with a cap 16 isillustrative for some embodiments only.

The MEMS device 10 may include on-chip circuitry to control and/ormonitor the structure. The circuitry has interconnects (not shown) toelectrically communicate with an external device, such as a computer.Alternatively, the MEMS device 10 may have structure only. In such case,the structure may communicate with off-chip circuitry for the notedpurposes.

Illustrative embodiments implement the MEMS device 10 as an inertialsensor, such as an accelerometer or a gyroscope. When implemented as anaccelerometer, the structure includes a normally stable (movable) masssuspended above the substrate, and circuitry (not shown but noted above)for detecting mass movement. Exemplary MEMS accelerometers include thosedistributed and patented by Analog Devices, Inc. of Norwood, Mass. Amongothers, see U.S. Pat. No. 5,939,633, the disclosure of which isincorporated herein, in its entirety, by reference.

When implemented as a gyroscope, the MEMS device 10 has an oscillatingmass suspended above the substrate, and circuitry (not shown but notedabove) for actuating and detecting mass movement. Exemplary MEMSgyroscopes include those distributed and patented by Analog Devices,Inc. of Norwood, Mass. Among others, see U.S. Pat. No. 6,505,511, thedisclosure of which is incorporated herein, in its entirety, byreference.

Discussion of an inertial sensor, however, is exemplary and thus, notintended to limit various embodiments of the invention. Accordingly,principles of various embodiments may apply to methods of producingother types of MEMS devices, such as piezoelectric devices.

FIG. 2 schematically shows a process of thinning a device wafer 11 (alsoreferred to as a “MEMS wafer”) in accordance with one embodiment of theinvention. The MEMS wafer has MEMS structure and/or circuitry producedin accordance with conventional processes. The process begins at step200, in which a working surface 18 (see FIG. 3) is prepared to securethe MEMS wafer. More specifically, as a preliminary production step, asingle silicon wafer (i.e., the MEMS wafer) often is processed to havean array of individual MEMS devices. It should be noted, however, thatprinciples of the invention also apply MEMS wafers with a single MEMSdevice 10.

Illustrative embodiments thin the entire bottom surface 14 of the waferbefore the wafer is singulated. Accordingly, the method prepares asurface upon which the top face of the MEMS wafer may be secured (i.e.,the “working surface 18). Various embodiments use the working surface 18shown in FIG. 3, which has a plurality of openings 20 to receive theMEMS structure. If each MEMS device 10 includes both structure andcircuitry, then some embodiments may position/encapsulate only thestructure through the openings 20. Alternatively, both the circuitry andstructure may be encapsulated within the openings 20.

FIG. 4 is a plan view of an exemplary film frame 22 that may be used toproduce the working surface 18. The frame 22 may be constructed fromthin metal or plastic to define a generally circular opening having aperimeter. A thin plastic film 24 subsequently is mounted to the frame22 as illustrated in FIG. 5. As discussed below, the film 24 acts as acarrier for the MEMS wafer throughout the discussed MEMSproduction/thinning processes discussed below. Among other things, thefilm 24 may be a “Mylar” film having a thickness of approximately 5mils. In illustrative embodiments, the film 24 is coated with anadhesive on one side, thus enabling the film 24 to adhere to the surfaceof the film frame 22 and to the MEMS wafer.

To add the film 24, the film frame 22 may be placed on a pallet in afilm carrier station. The film carrier station has a roller of film 24positioned in a manner that permits the film 24 to be easily rolled ontothe film frame 22. Accordingly, the film 24 is rolled off the roller andplaced flat onto surface of the film frame 22. A rubber rolling pin maybe used to apply pressure to the film 24, thus forcing it to make goodcontact and adhere to the film frame surface. A knife can then cutaround the perimeter of film frame 22 to remove excess film 24. The filmframe 22 with the film 24 is considered to be a “film frame assembly26.”

After it is formed, the film frame assembly 26 may be transported to ahole punch station where it is placed on a pallet having an openinggenerally corresponding to the opening in the film frame 22. Thepunching station comprises a punch assembly for punching holes/openings20 in the film 24 as shown in FIG. 3. The punching station is programmedto punch holes/openings 20 in the film 24 in a programmablepredetermined pattern corresponding to the relative positions of themicrostructures on the wafer. The punch is selected to punchholes/openings 20 sized slightly larger than the individualmicrostructures.

After the working surface 18 is prepared, the method continues to step202, in which the MEMS wafer (i.e., having pre-formed MEMS structure) issecured to the working surface 18. At this step, it is important toalign the wafer with the holes/openings 20 so that the holes/openings 20match up with the microstructures. To those ends, the film frameassembly 26 is returned to the film carrier station and a second layerof film 28 (see FIG. 7), preferably a 3 mil thick “Mylar” film, isadhered to the first layer of film. The second layer of film has noopenings 20 and, therefore, seals one end of the openings 20.Alternately, the second layer of film can be added after the wafer ismounted to the film frame assembly 26.

The MEMS wafer then is fixed to the opposite side of the first layer offilm (i.e., the side where the openings 20 are still exposed). In someinstances, the MEMS structure of each MEMS device 10 on the wafer mayprotrude from the top surface 12 of the MEMS wafer and thus, extend intothe openings 20. In other embodiments, the MEMS structure may besubstantially flush with or below the top surface 12 of the MEMS wafer.

More specifically, to execute this step, the MEMS wafer may be placed ona chuck in a precision aligning and mounting station with the sidehaving the MEMS structure facing upwardly. A pair of cameras positionedabove the chuck obtains images of different areas of the wafer placed onthe chuck. The images are transferred to a pair of video screens or asplit screen on a single monitor. An operator observes the video imagesand aligns the wafer in the desired position. For instance, the videomonitor may have cross hairs that can be used for alignment purposes.The film frame assembly 26 then is inserted in a slot above the chuckwith one surface of the film frame assembly 26 facing downwardly. Thismounting causes the side of the first layer of film where the openings20 are still exposed to face downwardly toward the MEMS side of the MEMSwafer.

When the film frame assembly 26 is inserted into the machine, thecameras obtain images of the openings 20 in the film. The operator thenobserves the new images of the openings 20 and aligns them in a properorientation with respect to the MEMS wafer. In another embodiment of theinvention, however, the aligning station may be computer controlled andinclude pattern recognition software that automatically aligns theopenings 20 in the film frame assembly 26 with the MEMS wafer.

A rolling pin illustratively is not used to adhere the film to the wafersince rolling pin action could disturb the alignment or damage themicrostructure. Accordingly, in some embodiments of the invention, thealigning and mounting station is designed so that it can be sealed andevacuated to form a vacuum. The chuck containing the wafer then can bebrought into contact with the film so that the film readily adheres tothe MEMS wafer. In fact, in some embodiments, after evacuation andcontact between the film and the MEMS wafer, the chamber may bere-pressurized to atmospheric pressure, thus exerting a compressiveforce between the film and wafer. Such a compressive force should ensureadequate adhesion between the MEMS side of the MEMS wafer and the film.

In illustrative embodiments, there are no excess openings 20 outside ofthe MEMS wafer outline. Such excess openings 20 on the wafer edge couldinadvertently cause silicon slurry seep-in between the wafer and tapeduring thinning steps. Undesirably, such seep-in could lead to wafercracking.

For more information relating to steps 200, 202, and various otherprocesses discussed herein, see U.S. Pat. No. 5,362,681, assigned toAnalog Devices, Inc. of Norwood, Mass., entitled, “Method For SeparatingCircuit Dies From a Wafer,” and naming Carl M. Roberts, Lewis H. Long,and Paul A. Ruggerio as inventors. Also see US publication number US2002/0096743 A1, naming Timothy R. Spooner, Kieran P. Harney, David S.Courage, and John R. Martin as inventors, and entitled, “Method andDevice for Protecting Micro Electromechanical Systems Structures DuringDicing of a Wafer.” The disclosures of the noted patent and patentpublication are incorporated herein, in their entireties, by reference.

After the MEMS wafer is secured to the working surface 18, the processcontinues to step 204, in which its initial bottom surface issubstantially entirely removed to produce a final, substantially planar,bottom surface 14. As a result of this step, the overall profile of theMEMS wafer is thinner. To that end, at this point in the process, thetop surface 12 of the MEMS wafer is fixedly secured to the workingsurface 18. Standard processes then may be applied to the bottom surfaceof the MEMS wafer to produce the final bottom surface 14. In someembodiments, mechanical thinning may be applied with a conventionalbackgrinding device, such as a diamond grinding wheel.

In other embodiments, the initial bottom surface may be chemicallyremoved by conventional techniques, such as chemical etching processes.Exemplary chemicals that may be used in this process include potassiumhydroxide or tetra methyl ammonium hydroxide.

When using such surface removal processes (i.e., either or bothmechanical and chemical processes), it is important to ensure that theMEMS structure is sufficiently sealed within the openings 20 in the filmframe assembly 26. Such a seal should prevent debris from damaging thefragile MEMS structure on the MEMS wafer. Accordingly, the seal shouldprevent silicon dust penetration (e.g., when using a mechanical thinningprocess), chemical penetration (e.g., when using the chemical processes)or both. Of course, as noted above, mounting the MEMS structure withinthe openings 20 also ensures that the fragile MEMS structure does notbecome crushed during the thinning processes.

After the initial bottom surface is removed, the process may continue tostep 206, in which post-thinning processes may be performed. Amongothers, such post-thinning processes may include polishing the finalbottom surface 14 to remove surface imperfections, and dicing the waferto produce a plurality of individual MEMS devices. In addition, a sawtape may be adhered to the final bottom surface 14 to further protectthe wafer/individual MEMS devices when handled during subsequent steps.The film 24 and assembly 26 also may be removed by conventionalprocesses. For example, a tape or clip may be used to peel off thelayers of film. It should be noted that the order of these post-thinningsteps can vary. Each resulting MEMS device 10 can then be mounted in apackage assembly, on a board, or in some other conventional manner.

Various embodiments noted above thin the MEMS wafer before cappingbecause, among other reasons, it may reduce bond-line stresses arisingfrom wafer bow. Notwithstanding this consideration, other embodimentsmay thin the MEMS wafer after a cap wafer is secured to the MEMS wafer.FIG. 6 schematically shows one process for thinning a MEMS wafer coupledwith a cap wafer. Among other benefits, the cap 16 may further protectthe MEMS structure during a thinning step from chemicals and/or silicondebris.

FIG. 6 shows a first exemplary process of thinning/producing a cappedMEMS wafer in accordance with illustrative embodiments of the invention.The process begins at step 600, in which the working surface 18 isprepared. The working surface 18 illustratively is prepared insubstantially the same manner as discussed above with regard to step 200of FIG. 2. Before, during, or after step 200, the cap wafer may be diced(after it is secured to the MEMS wafer) in accordance with conventionalprocesses (step 202). This dicing step, however, does not dice theunderlying MEMS wafer—it dices the cap wafer only. As a result, the capwafer effectively forms a plurality of individual caps 16 on the MEMSwafer. For more information relating to capping, see U.S. patentpublication number 2003/0075794A1, the disclosure of which isincorporated herein, in its entirety, by reference.

The MEMS wafer then is secured to the working surface 18 in a similarmanner as discussed above with regard to step 202 of FIG. 2 (step 604).To that end, as shown in FIG. 7, the caps 16 each fit within one opening20 of the film frame assembly 26. More specifically, FIG. 7schematically shows a cross-sectional view of the MEMS wafer after it issecured to the working surface 18. As shown, the caps 16 fit withinopenings 20 formed by the pierced film. The top film 28 and pierced film24 together seal the cap 16 to protect it and its underlying MEMSstructure from debris. In a manner similar to other embodiments, if theMEMS wafer includes both structure and circuitry, then the circuitry maybe within the openings 20 or beneath the film. In either case, thecircuitry should be protected from environmental debris.

The process concludes in the same way as noted above with regard to theprocess in FIG. 2 by removing the initial bottom surface (step 606) andperforming post-thinning processes (step 608). Accordingly, the notedpost-thinning processes may be conducted to complete the MEMS productionprocess.

The inventors directed a test of the process of FIG. 6. To that end, thesemi-auto mode of the Okamoto GNX-200 backgrinder was used to grind theMEMS wafer to a desired thickness. Two six-inch wafers were prepared forbackgrinding to 13 and 19 mils, respectively. For this process, theinitial and final thickness for a six-inch wafer was adjusted.Specifically, the standard thickness using such a wafer is about 33 mils(i.e., 28 mils plus 5 mils tape/film). This was modified to 48 mils (28mils plus 15 mils for the cap 16 plus 5 mils for the film) toaccommodate the capped wafers. The wafers were successfully backgroundto the desired final thickness.

FIG. 8 shows another process for thinning a capped wafer in accordancewith illustrative embodiments. The process begins at step 800, in whichthe working surface 18 is prepared in a similar manner as discussedabove. In this case, however, there is no need to pierce the film toform the openings 20. Next, the cap wafer is secured to the workingsurface 18 (step 802) and the initial bottom surface of the MEMS waferis removed in a manner similar to the methods discussed above (step804). After the MEMS wafer is thinned, the cap wafer is diced in aconventional manner (step 806). This involves removing the cap waferfrom its contact with the working surface 18 and positioning the MEMSwafer on a saw device capable of performing singulating processes.Post-thinning processes then may be performed (step 808), thuscompleting the process.

Accordingly, as noted above, contrary to conventional knowledge, a MEMSwafer having pre-formed MEMS structure can be thinned to comply with thecontinuing need to reduce MEMS sizes. Such illustrative processesadequately protect the fragile underlying MEMS structure, thusfacilitating the overall process.

Although the above discussion discloses various exemplary embodiments ofthe invention, it should be apparent that those skilled in the art canmake various modifications that will achieve some of the advantages ofthe invention without departing from the true scope of the invention.

1. A method of producing a MEMS device, the method comprising: providinga device wafer having an initial bottom side and a plurality of movablestructures; coupling a cap wafer to the device wafer; singulating thecap wafer to form individual caps encapsulating the movable structureson the device wafer; fitting a protective plastic film having aplurality of clearance holes over the singulated cap wafer such that thecaps fit through the clearance holes; and after fitting the protectiveplastic film, removing substantially the entire initial bottom side ofthe device wafer to form a final bottom side.
 2. The method as definedby claim 1 wherein removing includes backgrinding the initial bottomside.
 3. The method as defined by claim 1 wherein removing includesusing a chemical to chemically etch the initial bottom side.
 4. Themethod as defined by claim 1 further including, after removingsubstantially the entire bottom side, removing the plastic film.
 5. Themethod as defined by claim 1 further comprising dicing the device waferinto individual MEMS devices.
 6. The MEMS device produced in accordancewith the method of claim 1.